Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 4/15/2005
Publication Date: 6/16/2005
Citation: Bosma, W.B., Bartelt, R.J., Momany, F.A. 2005. Calculations to determine the experimental conformation of a cyclic insect pheromone using a novel multi-faceted ab initio approach [abstract]. Midwest Theoretical Conference. p.10. Interpretive Summary:
Technical Abstract: A new pheromone from Galerucella calmariensis L. has recently been identified and has the chemical formula; 12,13-dimethyl-5,14-dioxabicyclo[9.2.1]tetradeca-1(13),11-dien-4-one. The structure was determined by mass spectrometry, NMR, and UV spectroscopy. The ring is flexible even though the furan ring and ester groups tend to prevent free flexibility, and the preferred conformation is difficult to deduce from NMR data. A molecular mirror image symmetry based on the flat furan ring is noted and the dihedral angles around the flexible ring can be of opposite sign and create a mirror structure of the same energy as the original structure. Knowing the experimental proton and carbon chemical shifts and their assignments from NMR spectral analysis, it should be possible to deduce the experimentally observable structure by calculating by ab initio methods the isotropic 1H and 13C chemical shifts for different conformations of low energy. The isotropic chemical shifts were calculated using the GIAO (gauge invariant atomic orbitals) method based on structures optimized by the B3LYP/6-311++G** level of theory. Two conformations of low energy were found within less than 0.5 kcal/mol of each other and both had very small rms deviations from observed chemical shift values; (less than 0.13 ppm) for the hydrogen atoms, and (less than 6.3-6.7) for the carbon atoms. The transition state barriers between different energy minima were obtained using an eigenvalue following routine and new minimum energy conformations obtained by energy optimization from the transition states. Dihedral angle driving methods were also used to find transition states and to examine how driving individual dihedral angles through mirror image values changes the low energy conformations. Starting with the lowest energy conformation, molecular quantum dynamics was carried out at the 4-31G level of theory and transition to the second lowest energy conformation was found which persisted through many picoseconds. The path for the molecule to move between the two mirror conformations was illusive but a reasonable series of conformational transitions were found.